1. Field of the Invention
The present invention generally relates to a laser diode, and more particularly to a gain-coupled distributed-feedback laser diode.
2. Description of the Related Art
Currently, wavelength division multiplexing (WDM) techniques are employed in the backbone transmission system of a high-capacity optical communication network. WDM is a technique for multiplexing optical signals on the wavelength axis to improve the transmission capacity.
The ITU-T grid, which is one of the ITU-T Recommendations announced by the Telecommunication Standardization Sector of the International Telecommunication Union (ITU-T), strictly defines the wavelength and the wavelength interval used in WDM. For this reason, the oscillation wavelength of the laser used as a light source has to be precisely controlled so as to be in conformity with the TTU-T grid.
A refractive index modulated distribution-feedback laser having a quarter wave (xcex/4) phase-shift (hereinafter, referred to as a xcex/4 phase-shift refractive-index-coupled DFB laser) is typically used as a light source for optical communication.
FIG. 1 illustrates a conventional xcex/4 phase-shift refractive-index-coupled DFB laser in a cross-sectional view taken along the longitudinal axis of waveguide extending in the direction of light propagation.
The xcex/4 phase-shift refractive-index-coupled DFB laser 1100 has a diffraction grating 1108 as a refractive index modulating section. The diffraction grating 1108 is provided in the area in which the optical field exists, and it has a periodic refractive index modulation features along the longitudinal axis of optical waveguide (that is, an active layer 1104a).
Throughout the optical waveguide (i.e., the active layer 1104a), forward waves and backward waves are coupled by the diffraction grating (i.e., the refractive index modulating section) 1108, and a resonator is constituted as a whole, which causes laser oscillation.
The conventional xcex/4 phase-shift refractive-index-coupled DFB laser 1100 has a xcex/4 shift region 1108a in the center of the resonator for the purpose of causing single-mode laser oscillation in a stable manner at the Bragg wavelength defined by the period of the diffraction grating.
Since the conventional xcex/4 phase-shift refractive-index-coupled DFB laser 1100 is designed so as to oscillate at the Bragg wavelength, the oscillation wavelength can be easily controlled by adjusting the period of the diffracting grating.
However, in the conventional xcex/4 phase-shift refractive-index-coupled DFB laser 1100, stimulated emission is promoted in the xcex/4 shift region 1108a located near the center of the resonator, and consequently, spatial hole burning (SHB) occurs. Spatial hole burning is a phenomenon of local depletion of carrier.
Spatial hole burning (SHB) causes the laser characteristic, especially the characteristic of single wavelength, obtained under application of driving current much higher than the threshold level to deteriorate. For this reason, it is hard to say that the conventional xcex/4 phase-shift refractive-index-coupled DFB laser is the optimum wavelength-multiplexed light source.
Another problem in the conventional xcex/4 phase-shift refractive-index-coupled DFB laser 1100 is the difficulty of achieving a high light-emission efficiency because both ends of the resonator are covered with anti-reflective (AR) coating. Since light disperses at both end surfaces covered with AR coating, light can not be taken out from one end surface at high efficiency.
Furthermore, it is difficult for the conventional xcex/4 phase-shift refractive-index-coupled DFB laser 1100 to increase the normalized coupling factor (xcexaL), and therefore, it is weak for reflected return light. The normalized coupling factor is a product of the coupling factor xcexa, which corresponds to the reflectance per unit length of the resonator, and the length L of the resonator extending in the direction of light propagation. Increasing the normalized coupling factor is a tradeoff for reducing spatial hole burning (SHB). As the normalized coupling factor (xcexaL) increases, the intensity of light reflected inside the resonator increases, and greater quantity of light can be confined in the active layer. However, if the internal light intensity increases, the electric field within the resonator increases, and spatial hole burning (SHB) is more likely to occur, causing deterioration of the characteristic of single wavelength.
To avoid these problems in the conventional refractive-index-coupled DFB laser, it has been proposed to use a gain-coupled DFB laser having a distributed feedback structure based on gain modulation, instead of modulation of the index of refraction.
A gain-coupled DFB laser realizes distributed feedback of light based on periodic modulation of gain. The periodic gain modulation can be achieved by a periodic structure of the active layer repeated along the longitudinal axis of the wave guide (i.e., in the direction of light propagation), or by giving a periodicity to the electric current injected to the active layer. Such an arrangement allows the gain-coupled DFB laser to oscillate at the Bragg wavelength without providing a phase-shift region. The gain-coupled DFB laser not only facilitates regulation of oscillation wavelength, but also reduces the adverse influence of SHB.
Since the gain-coupled DFB laser is superior in achieving a single-wavelength characteristic, high-intensity output light can be obtained by providing high-reflective (HR) coating on one end surface of the resonator.
However, the above-described advantages are limited to a pure gain-coupled DFB laser, in which gain modulation solely exists without other components. As a realistic problem, it is very difficult to fabricate a pure gain-coupled DFB laser.
For example, xe2x80x9c16-Wavelength Gain-Coupled DFB Laser Array with Fine Tunabilityxe2x80x9d, LEEE Photonics Technology Letters, vol. 8, 22 (1996) discloses a device that modulates a gain using an active layer delineated into a periodic pattern by etching. However, since the active layer extends intermittently, the alternation of existence and absence of the active layer causes the index of refraction to be modulated during the gain modulation. Accordingly, a considerable amount of refractive-index-coupling component is produced inevitably, which can not be neglected in a pure gain-coupled DFB laser.
Another publication xe2x80x9cPurely Gain-Coupled Distributed Feedback Semiconductor Lasersxe2x80x9d, Y. Luo et al., Applied Physics Letters, vol. 56, 1620 (1990) discloses a technique for realizing a pure gain coupling using a pattern-providing layer. However, this technique requires strict and precise control of the thickness of each semiconductor film. This requirement reduces the degree of freedom for design of the laser structure.
The present invention was conceived in view of these problems existing in the current technique of a gain-coupled DFB laser, and it is an object of the present invention to provide a laser diode that is superior in the single-wavelength characteristic without causing much limitation to the freedom in design of the laser structure.
To achieve the object, a laser diode comprises a first cladding layer having a first conductivity, a second cladding layer having a second conductivity, an active layer located between the first cladding layer and the second cladding layer and extending from one end surface to the other end surface, a first electrode configured to inject a carrier with a first polarity into the active layer via the first cladding layer, and a second electrode configured to inject a carrier with a second polarity into the active layer via the second cladding layer. The active layer comprises a first active region and a second active region, which are arranged alternately and periodically from one end surface to the other end surface in the direction of light propagation, and the first active region and the second active region define type-II hetero junction between them.
This arrangement can efficiently prevent refractive-index-coupling, while maintaining superiority in single-wavelength characteristics. In addition, the degree of freedom for design of the laser structure is large.
Alternation of the first active region and the second active region is repeated at a period twice the Bragg period.
With this arrangement, even if a certain amount of refractive-index-coupling component remains in the distributed feedback effect, such a component is caused merely by a higher order (i.e., the second order) diffraction, and therefore, influence of the refractive-index-coupling component is very small. Consequently, a gain-coupled DFB laser diode capable of achieving almost pure gain coupling can be provided.
Preferably, a high-reflective film is provided to one end surface of the laser diode.
By proving a high-reflective film on one end surface, highly concentrated output light can be extracted from this end surface. Because the gain-coupled DFB laser diode according to the invention conducts stable oscillation at a single oscillation wavelength, the stable operation will not be disturbed by the mirror (i.e., the high-reflective film) formed on one end surface.
One of the first active region and the second active region is made of a compound semiconductor material containing antimony as a group V element. For example, the first active region is made of InAlGaAs, and the second active region is made of AlGaAsSb.
A dimension of the first active region along a longitudinal axis of the laser diode is substantially the same as that of the second active region along the longitudinal axis of the laser diode.
With this arrangement, the type-IT hetero junction, that is, a gain region is repeated at a single interval, and efficient gain coupling is realized.